The present invention relates to a glass prism and a method for producing the prism.
Recently, according to development in the technology of optical recording and optical communication, various types of small-sized prisms, which have the advantages for optical signal processing, such as transparency, low expansion rate, mass productivity, proper polish-ability and the like are used as an optical head of an optical disk device, an optical switch of the optical communication, or the like.
There are prisms in which light incident from the first surface is totally reflected by the second surface and exits from the third surface, light incident from the first surface is totally reflected by the second surface and then by third surface and exits from the first surface, and the like. To make correct use of the function of operating an optical signal like this, it is necessary that prism surfaces such as the first to third surfaces and the like which are subjected to the optical signal, are precisely formed at proper angles therebetween, and are mirror finished surfaces with high surface accuracy.
To satisfy the foregoing requirements, the glass prism is generally produced by the steps of preparing a glass flank approximately in the shape of a prism which is roughly formed with a processing margin, and carrying out polish finish after precisely grinding the prism surfaces one by one, in order to make the surfaces into the mirror finished surfaces having high angular accuracy (accuracy in angles between the prism surfaces) and high surface accuracy.
Recently, in accordance with increase in capacity of the optical recording and in speed and capacity of the optical communication, such an optical head for the optical disk device, an optical switch of the optical communication, and the like for their use grow in demand. Accordingly, an inexpensive resin prism is widely used, but there may be cases in which the resin prism cannot be used because of the reliability such as the temperature dependency of optical characteristics, moisture resistance, and the like.
When the foregoing conventional producing method is adopted to produce the objective glass prism, it is necessary to form the prism surfaces so as to have required high angular accuracy therebetween, surface roughness and flatness by carrying out the polish finish after precisely grinding the prism surfaces one by one. Therefore, many complicated producing processes are necessary, so that a cost rise due to low producing yield may become a problem.
In a case of producing a small-sized prism the cross sections of which, perpendicular to prism surfaces, are less than 100 mm2, or producing an elongate prism which satisfies the relation of L ≧1.5D/20.5, where D represents the diameter of a circumscribed circle of the cross section perpendicular to prism surfaces, and L represents the length of the prism in a direction parallel to the prism surfaces, the glass blank is hard to handle. Thus, there are problems that the glass blank is susceptible to damage during operation such as polishing, that it is difficult to form the prism surfaces with required high angular accuracy therebetween, and that it is difficult to form the prism surfaces into the mirror finished surfaces having uniform surface accuracy. There is even a case where the prism surfaces completed with much effort are damaged during processing or cleaning after processing. Furthermore, if a corner portion adjacent to the prism surfaces cracks during a polishing process or the like, resistance to breakage decreases, so that the blank is often damaged especially in the elongate prism and the yield is lowered. Low yield decreases efficiency in manufacturing, so that the conventional producing method of the glass prism is unsuitable for mass production.
In order to address the foregoing problems, Japanese Patent Laid-Open Publication No. Hei10-1321 has proposed a method wherein drawing a round glass bar to obtain a rod-shaped glass blank, reheating anti press-forming the glass blank into a polyhedral longer body to take out many prisms each having a predetermined length. According to this method, however, it is difficult to control a forming die determining the quality of the prism surfaces, so that there is a problem that the manufacturing cost will be less reduced when the forming die has a short life.